High speed extrusion

ABSTRACT

An apparatus for extrusion, in particular for extrusion of metal, comprising an extrusion body defining a passage for material to be extruded, including a die having a bearing opening extending between a bearing opening entry surface and a bearing opening exit surface, and wherein cooling means are arranged in the body defining the extrusion passage upstream of the bearing opening exit surface, the cooling means comprising the material of the body being differentiated in thermal conductivity so as to define cooling pathways.

RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national phase application of WO2006/028367 PCT/NL2005/000643 (WO 2006/028367), filed on Sep. 6, 2005,entitled “High Speed Extrusion”, which application claims priority toEuropean Application Serial No. EP 04077477.0, filed Sep. 6, 2004, eachof which is incorporated herein by reference in its entirety.

Extrusion is a manufacturing process in which sections or profileshaving constant cross-section are produced by placing feed stock, abillet, in a container and forcing it through a die opening by a ram.The shape and dimensions of the die determine the cross-section of theproduct. For most metals, this batch or semi-continuous process isoperated at elevated temperature to enhance the ductility of thematerial and/or to off-set mechanical loads.

Extrusion is a well-established means of manufacturing sections in awide variety of shapes and dimensions. Extrusion facilities are beingoperated throughout the industrialized world. Extrusion is applied toferrous (e.g. steel, stainless steel) as well as to non-ferrous (e.g.aluminium, copper, magnesium, titanium) metals and alloys. Aluminium ispresently by far the prevailing material for processing. So-calledsquare dies are usually used to produce solid sections. Single- andmulti-hollow sections require more intricate tooling constructions knownas spider dies, porthole dies and bridge dies. These dies enable thesplitting and rejoining of the metal flow around the supports of aninternal mandrel.

A principal limitation of the extrusion process for metals is theoccurrence of hot cracking, also known as hot shortness. This surfacedefect is initiated by incipient melting of the material in the plasticzone due to excessive temperature rise, which, in combination with themechanical stresses at the die bearing leads to cracks. This phenomenonis metal- and alloy specific, as it depends on such properties as flowstress and the incipient (i.e. non-equilibrium) melting temperature.Further, the higher the extrusion speed, the more adiabatic-like theprocess is, so in effect hot cracking fixes the speed for obtainingsound products to an upper level.

Since hot shortness is such a critical aspect in metal extrusion,considerable research and technological development effort has beendedicated to stretching this limit. With particular reference to theprocessing of aluminium and magnesium alloys, research has been done on:alloy compositions and billet preparation methods (e.g. casting,homogenization), alternative extrusion methods (e.g. indirect extrusion,hydrostatic extrusion) and adapted processing and tooling (e.g.closed-loop control systems for isothermal extrusion, cooling of dies).

FR 980 781 discloses an apparatus for extrusion having a body definingan extrusion passage and a die having a bearing opening with a coolingstructure in a body defining the extrusion passage.

Regarding the cooling of dies for metal extrusion, JP 3138018 and JP3138019 disclose an open system to cool extrusion dies by guiding acooling medium through the die to the clearance cavity behind the diebearing.

WO 9112097 describes a construction for the cooling of metal extrusiondies. The construction is used to guide a cooling medium through the diethrough feed passages, splitting in circumferential direction in adistribution passage and letting it escape closely after the die bearingin the clearance cavity.

JP 3138018, JP 3138019 and WO 9112097 disclose square dies in which thelatent heat for evaporation of a fluid is used for cooling in theclearance cavity behind the die bearing.

It is an object of the invention to improve the processing and toolingfor the extrusion of metals. In particular, it is an object of theinvention to improve the processing and tooling of metal extrusion suchthat the extrusion speed at which hot cracking is absent or at anacceptable level, can be increased.

By differentiating the material of the body in thermal conductivity soas to define cooling pathways, the relatively low thermal conductivityof the tooling material can be greatly increased.

Preferably, thermal conductivity is locally increased by providing agreen porous structure from powder, e.g. through powder printing,sintering the structure. By filling the pores at some locations withconductive material, a cooling pathway can be formed. By filling thepores at other locations with strengthening material, a strengtheningzone can be formed. By providing portions of open, porous structure, influidic communication with a cooling channel, porous cooling channelsportions can be provided.

The porous body can be manufactured by so called rapid prototyping/rapidmanufacturing techniques. Such techniques allow the apparatus to beprovided with a tailored cooling system.

By providing the porous structure with areas of increased density,boundaries can be defined e.g. for the cooling pathways, the (porous)cooling channels or the strengthening zones.

The body is preferably at least partially built up as a porousstructure. However, the body may also e.g. be provided with an insert tolocally differentiate the material of the body in thermal conductivity.

Preferably, the porous structure is built as a green structure fromparticles, e.g. metal and/or ceramic particles. The green structure cansubsequently be sintered to form a high-strength porous structure, e.g.one that can withstand filling the pores with the material having ahigher thermal conductivity and/or the material having the higherstrength. When the material having the higher thermal conductivity ise.g. copper, it is infused into the pores through capillary action.Likewise, the strengthen material may be molten steel.

The porous structure may subsequently be used as an insert when using aconventional die manufacturing technique, e.g. through sintering.

However, the apparatus may also be formed using a rapidprototyping/rapid manufacturing technique. Preferably, the porousstructure is then built up in layers through layer based manufacturing,preferably using sliced CAD computer files. Suitable rapid manufacturingtechniques are e.g. direct metal laser sintering made available by thecompany EOS, direct metal laser melting as made available by the companyTrumpf, and powder printing with infiltration as made available by thecompany Pro-metal.

By passing the surface of the material that is processed along a coolingzone before leaving the bearing opening peak temperature at criticallocations can be topped of so that the surface of the material to beprocessed can be prevented from reaching the incipient meltingtemperature. This way, extrusion can take place at higher speed, whileit is still possible to obtain sound products. Further, due to thereduction of peak temperature in mechanically loaded areas, the life ofthe die may be prolonged. In addition, due to reduction of diedeflection, the accuracy of the product can be improved.

By cooling the surface area at critical places before the surface hasleft the bearing opening, the formation of cracks at local surface areasweakened by melting can be prevented.

By cooling at least a part of the surface of the bearing opening of thedie, the cooling can be applied at the most critical areas.

By cooling at least a part of the surface of an entrance zone of thebearing opening, more area can be made available for cooling, and moretime can be made available for heat extraction from the moving materialto be processed.

In a preferred embodiment, the surface of the material to be processedis passed along an entrance zone that converges in the direction of theflow, so that relatively large area can be made available for cooling.In addition, the redundant work in the material to be processed can besimultaneously reduced, thereby reducing the rise in temperature of thematerial to be processed. Preferably, the entrance zone of the die isconical.

Further advantageous embodiments of the invention are discussed belowand are defined in the appended claims.

The invention shall be elucidated using a preferred embodiment shown ina drawing. In the drawing:

FIG. 1 shows a schematic cross-section of an apparatus for extrusionaccording to the invention;

FIG. 2 shows a schematic cross section of a first embodiment of a diebody that is differentiated in thermal conductivity according to theinvention;

FIG. 3 a shows a schematic cross section of a second embodiment of a dieaccording to the invention; and

FIG. 3 b shows a detail of the porous structure of the die of FIG. 3 a.

In the figures, corresponding parts have been denoted using the samereference numerals. The figures only shows schematical representationsof exemplary embodiments, which are given as non-limiting examples ofthe invention.

FIG. 1 shows an apparatus 1 for extrusion of metal. The apparatuscomprises a die 2 having a bearing opening 3 through which metal to beprocessed is forced, so that it is formed into a profile having aconstant cross-section.

The bearing opening 3 comprises a bearing opening surface 3 a thatextends between a bearing opening entry plane 4 of the die and a bearingopening exit plane 5. The exit plane forms an opening of the die 3 thatis in downstream direction adjacent to a clearance cavity 6 formedbetween the outer surface 7 of the profile 8 and a clearance surface 9of the die.

The apparatus 1 further comprises a container 10 that is closed off bythe die 2. The container 10 holds a billet 11 of metal to be processedin a chamber 12. The apparatus 1 further comprises a ram 13 that can bepushed into the chamber 12 to force the material of the billet 11through the bearing opening 3 of the die 2, thereby forming the profile8. The extrusion process may also be arranged differently, e.g. in ahydrostatic variant wherein the chamber is filled with a liquid ontowhich the ram 13 acts to indirectly force the billet 11 through thebearing opening 3 of the die 2.

The apparatus is provided with cooling means 14 that are located streamupward of the exit surface 5 of the bearing opening 3.

In this arrangement, the cooling means comprises cooling channels 15 ofa cooling circuit that extends along an entrance zone 16 of the die 2.In this embodiment, the die 2 is of modular construction having anentrance ring 2A placed between the container 10 and theproduct-specific part of the die comprising the bearing opening. Theentrance ring defines an entrance zone that converges conically in thedirection of the entrance surface 4 of the bearing opening 3. Togetherwith a die holder 2B, the entrance ring holds the product-specific partof the die 2 that is provided with the bearing opening. It shall beclear that the cooling means can be arranged to extend along at least apart of the surface of the bearing opening and/or at least a part of thesurface of the entrance zone.

In this embodiment, the cooling zone is thus placed at least partiallyoutside of the container.

The conical shape of the die entrance opening reduces the amount of heatgenerated within the critical areas of the billet.

Further, by cooling stream upward of the exit plane of the bearingopening 3, heat can be conducted from the critical areas of the billet11. In this embodiment, this is done by passing or circulating a coolingmedium through channels 15 in the die 2. This can be regarded as a meansof increasing the heat capacity of the die 2. The conical shape of thedie entrance zone 16 ensures a large contact area and allows forsufficient interaction time between the cooled surface area and thematerial that is processed. This way, heat generation is limited, whilesimultaneously stimulating heat diffusion.

FIG. 2 shows a schematic cross section of an embodiment of an apparatusaccording the invention in which the body is formed by the die 2.

The material of the body of the die 2 is differentiated in thermalconductivity so that a cooling pathway 20 is defined. The die 2 is builtup of a high strength die part 21 of e.g. high strength steel. The bodyof the die 2 further comprises an insert 22 of highly conductivematerial, e.g. copper.

The body of the die 2 also comprises a supporting die part 23 in whichcooling channels 15 have been arranged.

FIG. 3 shows a schematic cross section of a second embodiment of a dieaccording to the invention. The die 2 has been manufactured by rapidprototyping and has been provided with a tailored cooling system.

The body of the die 2 has been built up from powdered steel particlesusing layer based manufacturing.

During manufacturing of the green structure, areas of increased densityhave been provided that define boundaries for cooling pathways, coolingchannels and high-strength areas.

After sintering, a cooling pathway 20 has been formed by filling thepores of the material between the boundaries with copper.

Strengthening zones 24 (shown in hatching) have been formed by fillingthe pores with steel. Also, cooling channels 15 for flowing a coolingliquid there through have been defined as void areas between boundariesof increased density.

The porous structures that have been left open as shown in FIG. 3 b, sothat cooling liquid may flow through a porous zone 25 in the structure.

It shall be clear that the invention is not limited to the embodimentdescribed above. For instance, the invention cannot only be applied tosolid sections as shown, but also to more complicated shaped sectionssuch as (multi)-hollow sections.

Preferably, the cooled die parts are thermally isolated from the heatedcontainer.

As discussed, the die construction can be modular, with components likea general purpose shaped cooled entrance ring, an exchangeableproduct-specific die, and a separate die casing for mechanical support.

The die can e.g. be manufactured by traditional means, or by using rapidmanufacturing techniques based on for instance layer deposition. Thelatter option has the advantage of providing the die with a tailoredcooling system, e.g. a conformal cooling system, and/or a distributionof the cooling channels that is adapted to the critical areas of thebearing opening and/or the geometrical features of the bearing opening.

Further, the die entrance can be of another shape that provides for alarge contact surface area for heat exchange.

Parts of the die, in particular the die entrance, can be provided with acoating. Such intermediary layer between billet and die material shouldbe able to withstand a thermal and mechanical load and give favourabletribological and/or interfacing conditions.

The cooling circuit can be laid out either to cool equally across thecircumference of the die or in a tailored fashion. The latter willenable to locally relieve critical locations (“hot spots”) and/or mayreduce residual stresses in and distortions (warping, twisting) of theproduced sections.

The cooling circuit can be close or open, meaning that the coolingmedium can either be circulating (involving its transport to and fromthe die) or escaping to the surroundings after having served. Bothoptions may use latent heat for evaporation of the cooling medium as anadditional cooling mechanism.

The cooling circuit can be provided with a control device. By doing so,the cooling action during the extrusion process and/or betweensubsequent extrusions can be adapted so as to level flash temperatureover the duration of the extrusion and/or within production runs.

The cooling circuit can be used to cool, but also to preheat the die.This may be beneficial during the start up of a production, and duringdown-time and/or to avoid chill of the billet.

Those variations will be clear to the skilled man and are within thescope of the invention as the defined in the appended claims.

1. An apparatus for extrusion, in particular for extrusion of metal,comprising: an extrusion body defining an extrusion passage for materialto be extruded, including a die having a bearing opening extendingbetween a bearing opening entry surface and a bearing opening exitsurface; and cooling means in the body defining the extrusion passageupstream of the bearing opening exit surface, the cooling meanscomprising the material of the body being differentiated in thermalconductivity so as to define cooling pathways, wherein the bodycomprises a porous structure comprising a plurality of pores, furthercomprising fluid pathways formed by open portions of the porousstructure of the body that are in fluidic communication with a coolingchannel.
 2. The apparatus of claim 1, further comprising the coolingpathways comprising zones of increased thermal conductivity which havebeen formed by filling at least a portion of the pores of the porousstructure with material having a higher thermal conductivity than amaterial defining the porous structure.
 3. The apparatus of claim 2,wherein the material of increased thermal conductivity comprises copper.4. The apparatus of claim 2, further comprising at least a portion ofthe pores of the porous structure being filled with material having ahigher strength than the material defining the porous structure to formzones of increased strength.
 5. The apparatus of claim 1, wherein thecooling means further comprises cooling channels arranged for the flowof a cooling medium there through.
 6. The apparatus of claim 1, furthercomprising at least a portion of the pores of the porous structure beingfilled with material having a higher strength than the material definingthe porous structure to form zones of increased strength.
 7. Theapparatus of claim 1, wherein the porous structure comprises sinteredparticles.
 8. The apparatus claim 1, wherein the porous structurecomprises areas of increased density that define boundaries for thefluid pathways.
 9. The apparatus of claim 1, wherein the porousstructure is built up of layers.
 10. The apparatus of claim 1, whereinthe porous structure comprises steel.
 11. The apparatus of claim 1,wherein the cooling means are arranged to cool at least a part of thesurface of the bearing opening of the die.
 12. The apparatus of claim 1,wherein the cooling means is operatively associated with at least a partof a surface of the bearing opening.
 13. The apparatus of claim 1,further comprising the extrusion body defining an entrance zone thatconverges towards the bearing opening.
 14. The apparatus of claim 1,wherein the entrance zone is conical.
 15. The apparatus of claim 1,wherein the die is integrally formed with the extrusion body.
 16. Anapparatus for extrusion, in particular for extrusion of metal,comprising: an extrusion body defining an extrusion passage for materialto be extruded, including a die having a bearing opening extendingbetween a bearing opening entry surface and a bearing opening exitsurface; and cooling means arranged in the body defining the extrusionpassage upstream of the bearing opening exit surface, the cooling meansbeing configured to provide tailored cooling about an inner surface ofat least one of the extrusion passage and the die, wherein the bodycomprises a porous structure comprising a plurality of pores, furthercomprising fluid pathways formed by open portions of the porousstructure of the body that are in fluidic communication with a coolingchannel.
 17. The apparatus of claim 16, further comprising the coolingmeans comprising cooling pathways comprising zones of increased thermalconductivity which have been formed by filling at least a portion of thepores of the porous structure with a material having a higher thermalconductivity than a material defining the porous structure.
 18. Theapparatus of claim 17 further comprising the cooling means comprisingthe fluid pathways being formed by at least a portion of the pores ofthe porous structure remaining open to provide for the flow of a coolingmedium there through.
 19. The apparatus of claim 17, further comprisingat least a portion of the pores of the porous structure being filledwith material having a higher strength than the material defining theporous structure to form zones of increased strength.
 20. The apparatusof claim 16, wherein the porous structure comprises sintered particles.21. The apparatus of claim 16, wherein the die is manufactured by rapidprototyping.
 22. A method for manufacturing an apparatus for extrusioncomprising: forming an extrusion body that defines a material passagefor material to be extruded; forming a die communicating with thematerial passage, the die comprising a bearing opening extending betweena bearing opening entry surface and a bearing opening exit surface; andforming cooling means operatively associated with the extrusion bodymaterial passage upstream of the bearing opening exit surface bydifferentiating the material of the extrusion body in thermalconductivity so as to define cooling pathways, wherein the bodycomprises a porous structure comprising a plurality of pores, furthercomprising forming the cooling channels in the extrusion bodyoperatively associated with the cooling pathways for the flow of acooling medium there through.
 23. The method of claim 22, furthercomprising forming the cooling pathways by filling at least a portion ofthe pores with material having a higher thermal conductivity than thematerial of the porous structure.
 24. The method of claim 22, furthercomprising forming zones of increased strength in the extrusion body byfilling at least a portion of the pores with material having a higherstrength than the material of the porous structure.
 25. The method ofclaim 22, further comprising forming the porous structure as a greenstructure from particles.
 26. The method of claim 25, wherein theparticles comprise at least one of metal and ceramics.
 27. The method ofclaim 25, further comprising sintering the green structure.
 28. Themethod of claim 25, further comprising building up the green structurein layers.
 29. The method of claim 25, further comprising manufacturingthe green structure using a rapid prototyping/rapid manufacturingtechnique.
 30. The method of claim 22, further comprising forming thedie integrally with the extrusion body.